Is homosexuality caused by genetic or environmental factors? Is there a "gay gene"?

This debate is well into its second decade and we are still not sure. In 1993, Dr Dean Hamer and four colleagues from the US National Institutes of Health in Bethesda, Maryland published a study in the 16 July issue of Science on 114 families with homosexual men.

The study found linked DNA markers on the Y (male) chromosome. This suggested the possibility of genetics being involved in sexual preference of males. Thus a "gay gene" or genes was a possibility, but the evidence applied only to males.

Intense debate ensued both inside and outside the scientific community and continues today, although perhaps not as hot as it was a decade ago. Some argue that, whether or not a "gay gene" or genes exist, gays will be stigmatised with such a finding. Others argue that gays will be stigmatised without it.

Studies of male twins have suggested that about 50 per cent of the variability of sexual orientation is due to genes. This would leave about 50 per cent due to various environmental factors. In the March 2005 issue of Human Genetics, Dr Brian Mustanski and five colleagues including Hamer, studied the genetics of 146 families with two or more gay brothers. This study found that, among 60 per cent of the gay men, an identical clustering of genetic patterns occurred on three of the body's 23 chromosomes (7, 8, and 10).

This is somewhat greater than chance would predict, thus the existence of a "gay gene" or genes seems more likely.

In the 30 April 2005 British Medical Journal Dr Timothy Murphy, professor of philosophy and biomedical sciences at the University of Illinois in Chicago, comments that: "Like the others before it, this study is far from conclusive, but it adds to the growing sense that genes play a role in male sexual orientation. The evidence for a genetic contribution to female homosexuality is less well developed, but the case is hardly closed." ®

Stephen Juan, Ph.D. is an anthropologist at the University of Sydney. Email your Odd Body questions to s.juan@edfac.usyd.edu.au